Semi-conducting detectors, such as silicon detectors, are widely used in x-ray imaging to detect x-ray photons and convert them into digital signals or digital images by collecting charge carriers released by each interacting photon. For low-energy x-rays, semi-conducting detectors are commonly made to be pixelated, by orienting detector surface towards x-rays and dividing the detector plane into a number of pixels. Each pixel collects charges generated by x-ray interactions within the pixel through an applied electric field and feed the charges to corresponding front-end electronics. However, due to low atomic number and low density, for imaging with high-energy x-rays, such as x-ray computed tomography, the pixelated configuration of e.g. a silicon detector is not capable of capturing all high-energy x-rays with a substrate thickness less than half millimeter, resulting in low detection efficiency. The low atomic number of silicon also means that the fraction of Compton scattered x-ray photons in the detector dominates over the photo absorbed photons which creates problem because the scattered photons may induce signals in other pixels in the detector which will be equivalent to noise in those pixels.
One way to overcome the problem of low absorption efficiency for silicon is to employ edge-on configuration of the detector by orienting the edge of a silicon detector towards incident x-rays so that the depth of silicon can be significantly increased. The first mentioning of crystalline silicon strip detectors in edge-on geometry as an x-ray detector is R. Nowotny: “Application of Si-microstrip-detectors in medicine and structural analysis” Nuclear Instruments and Methods in Physics Research 226 (1984) 34-39. It concludes that silicon will work at low energies such as for breast imaging but not for higher energies such as computed tomography mainly because of the higher fraction of Compton scattering and problems related to this.
The edge-on geometry for semiconductor detectors is also suggested in U.S. Pat. No. 4,937,453 Robert Nelson “X-ray detector for radiographic imaging”, U.S. Pat. No. 5,434,417 David Nygren “High resolution energy-sensitive digital X-ray” and U.S. Pat. No. 7,291,841 B2 Robert Nelson et al. “Device and system for enhanced SPECT, PET, and Compton scatter imaging in nuclear medicine”. In U.S. Pat. No. 5,434,417, the concept of a segmented silicon strip detector is described, but how the charge collecting electrodes are connected to the front-end electronics and the arrangement of routing traces are not presented. In U.S. Pat. No. 7,291,841 B2 edge-on detectors are used for so called Compton imaging which does not relate to the present invention. In a paper by Shoichi Yoshida, Takashi Ohsugi “Application of silicon strip detectors to X-ray computed tomography” Nuclear Instruments and Methods in Physics Research A 541 (2005) 412-420 an implementation of the edge-on concept is outlined. In this implementation thin tungsten plates placed between edge-on silicon strip detector reduces the background of scattered X-rays and improve the image contrast with low dose.
The implementation of edge-on silicon strip detectors is further described in U.S. Pat. No. 8,183,535 B2 Mats Danielsson et al. “Silicon detector assembly for x-ray imaging”, Cheng Xu et al.: “Energy resolution of a segmented silicon strip detector for photon-counting spectral CT” Nuclear Instruments and Methods in Physics Research 715(2013)11-17 and Xuejin Liu et al.: “Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications” Journal of Medical Imaging 2(3) (2015) 033502. And as described each strip is further divided into depth segments to mitigate the load of front-end electronics under high flux applications of x-ray imaging, such as x-ray computed tomography. The photon-converted signals detected in each depth segment are conducted to an individual processing channel of the front-end electronics via a routing trace which connects the corresponding charge collecting electrode of the depth segment and the input of a channel in front-end electronics. Described by Cheng Xu et al.: “Energy resolution of a segmented silicon strip detector for photon-counting spectral CT” Nuclear Instruments and Methods in Physics Research 715(2013)11-17, two different metal layers are implemented on the charge collecting side of silicon substrate with one metal layer for charge collecting electrodes and the other layer for routing traces. In Swedish Patent Application No. 9801677-7, Mats Danielsson, a silicon detector with routing traces running in between charge collecting electrodes for monitoring of radiation therapy is described. E. BELAU et al. “Charge collection in silicon strip detectors” Nuclear instruments and methods in physics research 214(2-3) (1983) 253-260 described a telescope detector used for particle physics with routing traces on the same layer as the charge collecting electrodes. Thin and heavy metal sheets are suggested in U.S. Pat. No. 8,183,535 B2 Mats Danielsson et al. “Silicon detector assembly for x-ray imaging” to be attached to the subsets of silicon detectors to partly stop the scattered photons as a result of Compton scattering from reaching other silicon detectors.
Capacitance is one of the most critical parameters for a detector, which is directly related to the level of electronic noise of front-end electronics. The noise level needs to be as low as possible for any imaging detectors to acquire images with satisfactory quality. For energy-integrating detectors, in which the detected photons are integrated over a certain time interval, the electronic noise is integrated in the readout signal, resulting in deteriorated image quality. For photon-counting detectors, the lowest energy threshold should be set higher than the level of noise floor to reject fake counts induced by noise. Therefore, lowering the input capacitance to the front-end electronics is an important task when designing a new detector.
For semi-conducting detectors such as silicon detectors, taking the single-sided silicon strip detector as a particular example, the following two contributions are the main capacitance sources, the backside capacitance which is the capacitance between a charge collecting electrode and the backside of the silicon substrate, and the inter-strip capacitance which is the capacitance between neighboring charge collecting electrodes. For detectors with routing traces connecting the charge collecting electrodes and the input of front-end electronics, the trace capacitance also plays a significant role, including the capacitance between neighboring routing traces, the capacitance between a routing trace and a charge collecting electrode. Out of the range of the x-ray sensitive area, the silicon substrate might be covered by a large area of implantation layer which would also contribute to capacitance if there are routing traces running on top of that.
There has been a considerable interest in edge-on silicon detectors for medical imaging in particular for imaging with high-energy x-rays. However, when the number of depth segments is larger than one, a problem arises in conjunction with routing to the front-end electronics. A design with two metal layers is described by Cheng Xu et al.: “Energy resolution of a segmented silicon strip detector for photon-counting spectral CT” Nuclear Instruments and Methods in Physics Research 715(2013)11-17, where the routing traces run on a different metal layer than that of charge collecting metal electrodes, exhibiting higher capacitance and complexity in implementation. And more dead space is added by the insulator material between two metal layers, which results in a loss in geometrical efficiency. It is also suggested in U.S. Pat. No. 8,183,535 B2 Mats Danielsson et al. “Silicon detector assembly for x-ray imaging” to spread out the front-end electrodes over the area of the sensor or on top of the sensor, with which the front-end electronics would be exposed to radiation and also the front-end electronics would take space and make impossible a very dense packing. Therefore, it is desirable to provide a way to conduct routing traces to the front-end electronics with reduced implementation complexity and optimal capacitance to the front-end electronics.